Help with delayed circuits

Discussion in 'The Projects Forum' started by nofrills, Apr 5, 2009.

  1. nofrills

    Thread Starter New Member

    Apr 5, 2009
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    Hi everybody, I'm fairly new to the game so please forgive me if my terminology is a little off..

    Anyway, I'm installing three aftermarket gauges in a car (12v) - theyre the type that are black-faced until the engine is switched on. Normally i'd have no trouble wiring them up to turn on simultaneously as the key reaches the 'on' position. However, just for fun, I'd like to have them all switch on one after the other instead of at the same time. I'm thinking half a second delay or something in that vicinity

    Here is where my lack of terminology comes in - I realise that I will probably need some sort of timer to fit to the second and third circuits to effect the delay, but I have no idea exactly what to ask for. Alternatively there may be a cheaper way to do it that I haven't thought of.

    It's probably a simple answer for most of you. But if anyone can tell me what I should be looking for, that would be greatly appreciated!
     
  2. Mike2545

    Active Member

    Mar 26, 2009
    116
    3
    How much current do the gauges draw?
     
  3. nofrills

    Thread Starter New Member

    Apr 5, 2009
    4
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    Hi, thanks for the reply - I don't have the guages with me to go off of, unfortunately, but they are all mechanical-type. In other words, the only current they will be drawing would be enough for the single 12v bulb in each to light up. Is that enough information? I can have a quick look online and see if I can find some other specs for them, but no guarantee. Cheers
     
  4. Mike2545

    Active Member

    Mar 26, 2009
    116
    3
    You could do something like this:
     
  5. nofrills

    Thread Starter New Member

    Apr 5, 2009
    4
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    thanks for that, im still getting my head around it.. correct me if i'm wrong here, but:

    1.the relay is not powered until the transistor is switched
    2.the transistor only switches once the base recieves sufficient negative voltage
    3.negative voltage is generated by the capacitor in circuit with the resistors?

    im just having a little trouble working out why there is a normal resistor and a variable resistor?
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Correct.
    When the base of the PNP transistor becomes about -0.63v in respect to the emitter, it will begin to conduct.
    Actually, the resistors slowly charge the capacitor to a voltage potential that is negative compared to the transistor's base. Unfortunately, the values of resistance are so high that a standard PNP resistor probably wouldn't have sufficient gain to supply enough current through the relay's coil to cause it to become energized. Also, when the circuit is de-energized, the relay will produce a high-voltage spike in reverse polarity, because there is no diode across the coil connections.

    That is so you can adjust the RC time constant. Unfortunately, even 100k by itself is too much resistance to get the transistor anywhere near saturation; and the lower the value of resistance you use, the shorter the RC time constant - meaning you'll need a larger capacitor.

    Also, the capacitor will take a very long time to fully discharge through the base/emitter junction.

    I'm afraid it's not a circuit that has much practical use; it's just a lab experiment kind of thing.
     
  7. nofrills

    Thread Starter New Member

    Apr 5, 2009
    4
    0
    hmm thanks for the reply guys, it looks like i should probably try looking for a premade kit - apparently you can get delay systems for use with amps and subs in cars, perhaps i can find something useful there. strange really - its a simple concept, but theres really no easy way to go about it
     
  8. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    Here is one that is similar to the relay above, but uses an SCR:
     
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